Is the main radiation danger the Sun or the background "cosmic" radiation?

I've been assuming the latter, but I realize now that I don't actually know.

Both are significant dangers, but radiation from the sun can be shielded fairly easily. Not so high energy cosmic rays.

You can for example put an entire planet between you and the sun radiation by going out at night.

Doesn't matter too much for the rocks: if a few metre of rock won't absorb the radiation, neither will your body.

That doesn't make too much sense to me, since your body could be harmed by absorbing only a fraction of the energy of a large radiation dose.

Valid point. Though I think given the spectra involved, it probably doesn't apply.

Underground living on Mercury is much more interesting than Mars. (There's a band close to the poles and a few meters down that has very stable and comfortable temperatures.

Perhaps if you're building a cellar. If you want to make a 10-story building - even in a virgin terrain, it's a complicated operation.

Not to mention that you need to make the materials such that they resist a huge side pressure, water leaks and so on.

Psychological costs are also costs.

Water leaks are not going to be a problem on mars. Significantly lower gravity 3.711 m/s vs 9.8m/s also drastically lowers pressure from burrowing.

Wide-open spaces, even enclosed ones, can reduce the psychological burden of confinement quite a bit -- there's a variety of Russian strategic-missile submarine with a swimming pool, for example. This subject also comes up frequently in SF (science fiction, not San Francisco, which has the opposite problem); there was some discussion of it on the "Atomic Rocket" worldbuilding guide, IIRC.

I… I think the picture of that "pool" in your head is slightly wrong [1].

Tangential: the source [2] is pretty interesting.

[1]: http://pics.livejournal.com/igor113/pic/001e551p

[2]: http://igor113.livejournal.com/27205.html

Why build layers at all? Urban concentrations too high?

Is it possible to make a huge nuclear explosion/reaction that produces the same atmosphere we have on earth?

There are ways to teraform the planet, but they are well beyond our capabilities.

No?

Wrong grammar. I was actually asking a question.

how to become ant people

Human evolution would surely change direction after a few millenia of living underground in lower gravity